started adding django 2.2 support

Author: Asif Saif Uddin

tox.ini

@@ -1,8 +1,8 @@
 [tox]
 envlist =
-  {py27,pypy,py34}-django{16,17,18,19,110,111}
-  py35-django{19,110,111,-master}
-  py36-django{111,-master}
+  {py27,pypy,py34}-django{111}
+  py35-django{20,22,111,-master}
+  py36-django{111,22,-master}
   docs
 
 [testenv]
@@ -16,11 +16,7 @@ basepython =
 deps =
   py{27,py}: mock
   py{27,py}: futures
-  django16: Django>=1.6,<1.7
-  django17: Django>=1.7,<1.8
-  django18: Django>=1.8,<1.9
-  django19: Django>=1.9,<1.10
-  django110: Django>=1.10,<1.11
+  django22: Django>=2.2.1,<3.0
   django111: Django>=1.11,<1.12
   django20: Django>=2.0,<2.1
   django-master: https://github.com/django/django/archive/master.tar.gz